1. Field of the Invention
The present invention generally relates to communication networks and more particularly to equalizer training.
2. Related Art
Communications networks have become quite common place in office environment and are slowly finding more and more applications at homes. Conventionally, communications networks communicate through communications links, such as T1 lines, cable lines, telephone lines and the like. For example, home networking may be accomplished via standard phone lines as transmission media. Typically, a communications network has several devices connected to it. For instance, a local area network (“LAN”) 101 shown in FIG. 1 can have numerous devices attached to it, such as personal computer 102, printer 106, personal digital assistant (“PDA”) 104, and laptop 108.
When a communications network, such as LAN 101 has many devices connected, the devices may interfere with receive signals intended for another device and, for example, may echo in such receive signals. For instance, when personal computer 102 sends signal 110 to an intended receiver, for example, laptop 108. Signal 110 is sent over the transmission medium, for example, home telephone line. The transmission medium for LAN 101 is a broadcast channel, in which other connected devices, such as PDA 104 may receive the signal, as shown by signal 112. As with any electrical transmissions circuit, if an impedance mismatch exists between the transmission medium and PDA 104, then signal 112 may be reflected, as shown by signal 114. Accordingly, received signal 116 would contain reflections when it reaches laptop 108, and such reflections or echoes may cause errors in the receive signal, which would increase the number of transmission errors.
Another common source of signal distortion is intersymbol interference (“ISI”), which is typically caused by the transmission medium of LAN 101. Typical wired transmission media, such as the twisted pair phone wiring used in LAN 101, have frequency dependent dispersion, and thus are typically band-limited. According to known digital communications theories, a band-limited transmission medium effectively disperses transmitted symbols in time. In other words, if an impulse signal is sent through a band-limited transmission medium it will be dispersed in time when it is received. ISI occurs when the pulse response of a band-limited transmission medium is longer in duration than the duration between transmitted symbols.
Dispersion from a band-limited medium causes a symbol to overlap with neighboring symbols. For example, when a transmitted symbol is a pulse, such pulse is dispersed in time and a transient portion of the signal exists before and after the transmitted pulse. The transient portions interfere with adjacent symbols, thus distorting adjacent symbols. The transient portion before the transmitted symbol is known as pre-cursor ISI and the transient portion after the pulse is transmitted is known as post-cursor ISI. In general, ISI becomes more problematic in high data rate systems because the duration between successive symbols is shorter.
Furthermore, symbols are typically distinguished from one another by symbol's respective voltage level. For example, a first symbol can be distinguished by a “+1” voltage level, while a second symbol can be distinguished by a “−1” voltage level. The effects of ISI may be observed when viewing an eye-pattern diagram of a transmitted signal on an oscilloscope. Normally, an eye-pattern diagram is substantially open when ISI is not present. In that case, a symbol is less susceptible to being misinterpreted for another symbol because of random noise. When the voltage levels of transmitted symbols are far apart from each other, each symbol is easily distinguished and, as a result, symbols are less susceptible to random noise, which may be caused by any number of noise sources, such as motorized appliances, i.e. vacuum cleaners, spark plug ignitions and the like.
On the other hand, when ISI is present, the eye-pattern diagram is closed. As such, the transmitted symbols are closer together in magnitude. As a result, symbols are less tolerant to random noise, because any interference reduces the minimum voltage level distance from an another symbol. For example, when ISI is not present, a first symbol is received with a voltage level of +1 Volts and a second symbol is received with a voltage level of −1 Volts. When ISI is present, the first symbol may be received with +0.3 Volts and the second symbol may be received with a voltage level of −0.3 Volts. If at one instance, the additive random noise contributes a +0.4 Volts to the second symbol, the resultant symbol will be 0.1 Volts. In such exemplary binary system, positive voltage levels are interpreted as the first symbol and negative voltage levels are interpreted as the second symbol and, thus, the random noise in conjunction with ISI create a symbol error. Had ISI not been present, the random noise alone would not create a symbol error.
To mitigate the distortive effects of ISI and echoes, an adaptive equalizer may be used. Adaptive equalizers can accommodate time-varying conditions of transmission medium. Also, an adaptive equalizer can estimate a model of the distortive effects of ISI and echoes in the transmission medium. Once an accurate model of the interference is ascertained, the adaptive equalizer may undo the distortive effects of the transmission medium. In order to assist the adaptive equalizer to estimate a model of the interference, a transmitter and receiver may share a common known sequence between each other. In such a scheme, the transmitter transmits the common sequence to the receiver. Since the receiver knows exactly how the unperturbed sequence should appear before being disturbed by the communications link, the adaptive equalizer is able to use the known sequence as a reference to estimate the distortion. Using a known sequence to undo the distortive effects of transmission medium of LAN 101 is a common bootstrapping method used in digital communications. Such known sequence, when transmitted at the start of a packet, is commonly referred to as a preamble.
The preamble is conventionally contained at the beginning of each transmitted packet and is used by the adaptive equalizer to train to a known sequence before processing the transmitted data contained in the transmitted packet. In any given communication protocol, all packet transmissions follow a known packet structure. FIG. 2 illustrates an exemplary packet structure. As shown, packet 200 contains preamble 210, header 212 and payload 214. Preamble 210 contains a known sequence, which is used to train an adaptive equalizer. For example, preamble 210 may be transmitted at two (2) mega-samples per second (“MSPS”).
As further shown, packet 200 also comprises header 212, which may include information such as the modulation type and symbol rate for the payload 214. Modulation type may indicate various modulation techniques, such as QAM (Quadrature Amplitude Modulation), PSK (Phase Shift Keying) and the like, which may used for modulating payload 214 data. The Header 212 is commonly transmitted with a predetermined modulation type and symbol rate referred to as the “base symbol rate”. The base symbol rate may not necessarily be the same as the payload symbol rate. Further, payload 214 contains the data of the transmitting device and can be transmitted at a different symbol rate from the base symbol rate, most conveniently at an integral multiple of the base symbol rate (2×, 3×, 4, etc.). The possible payload symbol rates are generally defined in a system specification. For example, a system specification could allow the payload 214 to be either equal to the base symbol rate or twice the base symbol rate. For example, if the base symbol rate is 2 MSPS, the payload could be transmitted at two (2) MSPS or four (4) MSPS.
Conventional adaptive equalizers are unable to accommodate various payload symbol rates and suffer substantial performance degradation as a result. For example, conventionally, when a packet is received that has the payload transmitted at two (2) MSPS, the adaptive equalizer can be trained by preamble 210, which was transmitted at two (2) MSPS; however, when the payload is transmitted at four (4) MSPS, the adaptive equalizer may not perform well since it was trained at two (2) MSPS. There is therefore an intense need in the art for methods and systems that are capable of training equalizers at proper symbol rates to improve performance.